The present invention is in the field of digital radiography. The invention more specifically relates to an image processing method applied for the purpose of enhancing the quality of diagnostic evaluation of a radiographic image.
In the field of digital radiography, a wide variety of image acquisition techniques have been developed such as computerised tomography, nuclear magnetic resonance, ultrasound, detection of a radiation image by means of a CCD sensor or a video camera, radiographic film scanning etc. By means of these techniques, a digital representation of a radiographic image is obtained.
In still another technique, a radiation image, for example an image of x-rays transmitted by an object, is stored in a screen comprising a photostimulable phosphor such as one of the phosphors described in European patent publication 503 702 published on Sep. 16, 1992 and U.S. Ser. No. 07/842,603. The technique for reading out the stored radiation image consists of scanning the screen with stimulating radiation, such as laser light of the appropriate wavelength, detecting the light emitted upon stimulation and converting the emitted light into an electric representation, for example, by means of a photomultiplier and finally digitizing the signal.
The digital images obtained by one of the acquisition techniques described hereinbefore can be subjected to a wide variety of image processing techniques.
If the unprocessed original image representation. is stored, it can be subjected off-line to different types of image processing techniques as well as to processing using different values of the processing parameters as frequently as required.
The processed or unprocessed images can further be applied to a display device and/or can be reproduced by means of an image recorder such as a laser recorder or the like.
For the purpose of display and/or hard copy recording signal, values are converted into density values envisioned in the display or hard copy according to a signal-to-density mapping curve that has a predetermined shape in between a minimum and a maximum displayable or reproducible density value.
In some applications radiologists protect their subjects against unnecessary exposure to X-rays by use of X-ray opaque xe2x80x98collimationxe2x80x99 material. The material is placed in the path of the X-ray beam so as to shield those areas of the patient which are not regarded as diagnostically important. Besides reducing patient dosage this technique has the additional advantage of limiting the amount of scattered radiation in the image field of interest. The regions of the resulting image which comprise the shadow cast by the X-ray opaque material (shadow regions) are exposed only by scattered radiation.
The presence of the collimation shadow region however can cause a problem in the display of the radiographic image on film or on a display device. Therein, the shadow region is relatively bright, and if displayed unmodified, may impair diagnosis of subtle lesions due to dazzle, especially if the unexposed region is relatively large.
It has been proposed in European patent application 523 771 to establish the region of interest and then to convert signal values of the radiographic image into density values to be displayed either as soft copy on a display device or as hard copy on film in such a way that pixels outside the diagnostically relevant zone are visualized so that the light transmitted by said image part when the hard copy is viewed on a console screen or when it is displayed, is effectively masked off.
In accordance with one embodiment described in this patent, the electrical signals of the pixels comprised within the diagnostically not relevant image zone within the radiographic image are converted to a uniform density values comprised between 0.5 and 2.5.
By applying this method, the dazzling effect produced by the light transmitted by the irrelevant region is decreased. However, as a consequence of the application of this method also the image information that was present in the diagnostically irrelevant area is lost.
It is an object of the present invention to provide a method of processing the radiographic image in a digital radiographic system in such a way that the dazzling effect described above does not occur and that the quality of diagnostic evaluation of a radiographic image is enhanced.
It is a further object of the invention to provide such a method wherein the information in a collimation region is not entirely lost.
It is still a further object to provide such a method for application in a system wherein a radiographic image is stored in a photostimulable phosphor screen and wherein said screen is scanned with stimulating irradiation, the light emitted upon irradiation is detected and converted into a digital signal representation.
Still further objects will become apparent from the description given hereafter.
The objects of the present invention are achieved by a method of processing an image in a radiographic imaging system wherein an electric signal representation of said image is mapped to density values for visualization as a hard or a soft copy image characterized in that the density of low density area in a diagnostically irrelevant zone in the image is enhanced and image structure in said zone is kept visible, by converting in said diagnostically irrelevant zone of the image pixels located at position (x,y) according to a conversion function g(x,y)=a.f(x,y)+(1xe2x88x92a).fmax wherein f(x,y) is the signal value before conversion of a pixel located at position (x,y), a is a value between zero and one, and fmax is equal to the maximum of values f(x,y), prior to being subjected to mapping into density values.
By applying the method according to this invention the pixel values of the radiographic image are converted in such a way that
(i) the pixels outside the diagnostically relevant zone are visualized or reproduced so that the light transmitted by said irrelevant image zone when the hard copy is viewed on a console screen or when the image is displayed, is attenuated, and
(ii) whereas the information within the diagnostically irrelevant zone is not lost.
This was obtained by processing pixels of the radiographic image differently when they were situated in the region of interest than when they were situated in the diagnostically irrelevant zone.
Pixels belonging to the irrelevant image zone where subjected to a conversion, for example being implemented in the form of a look up table, applied before signal-to-density mapping.
Pixels within the relevant image zone are converted by means of an identity transformation (i.e. they are not additionally converted) whereas pixels outside the diagnostically relevant zone are converted in accordance with a conversion function that can be represented by a straight line located above the identity transformation and that has a slope in between 0 and 1 so that pixels in the diagnostically irrelevant zone are converted to a higher average density than that of the remainder of the image and so that image information in that part of the image is not lost.
This conversion can generally be represented as: g(x,y)=af(x,y)+(1xe2x88x92a)fmax, wherein g(x,y) is a pixel value after transformation, f(x,y) is the pixel value before conversion and xe2x80x98axe2x80x99 is the slope of the conversion function applied to pixels outside the region of interest, fmax is the maximum of values f(x,y). The slope xe2x80x98axe2x80x99 has a value between zero and one and determines the minimum density to which pixel values in the diagnostically irrelevant zone can be converted.
In a preferred embodiment, the image is decomposed into a multi-resolution representation. That multiresolution representation is then modified for the purpose of enhancing the image quality, and the modified multi-resolution representation is finally subjected to a reconstruction process.
The additional signal conversion g(x,y) of pixels within the diagnostically irrelevant zone is applied to a partially reconstructed image, i.e. to an image obtained by applying the reconstruction process to modified detail images only up to a predetermined resolution level, i.e. detail images at coarse resolution levels.
The converted partially reconstructed image is then subjected to the remainder of the reconstruction process.
The predetermined resolution level is such that (1) the computation time is limited and (2) the dynamic range of the image in the diagnostically irrelevant zone approximates that of the original image so that important image structure is retained. Typically an image of 100.000 to 1.000.000 pixels is required for this purpose.
Finally signal values of the complete reconstructed image are mapped onto corresponding density values and are reproduced or displayed.
This embodiment is preferred since it provides that diagnostically irrelevant zones in the image are reproduced or displayed in a way so that they do not produce a dazzling effect but that important image structures in these zones remain visible.
Mapping of signal values onto density values has been described extensively in European patent application EU-A-546 600, the contents of which is incorporated herein by reference.
A hard copy image can be recorded for example by means of a laser printer onto a photographic material.
Signal values within a diagnostically relevant image zone are preferably mapped onto density values that are situated in between a minimum density value equal to the fog value of the photographic material and a maximum value situated in a density range from 1.5 to 3.
Often a reproduction also comprises a window wherein patient identification data and occasionally other data are recorded. Signal values pertaining to pixels within such a window are preferably mapped onto density values within 0.5 and 1.5.
Signal values representing pixels that are located outside the image are mapped onto the maximum of density values attainable on the hard or soft copy.
The method of the present invention can be applied when reproducing or displaying images acquired by a great number of acquisition devices as described in the introductory part of this application. It was however, specifically designed for application in a system in which a stimulable phosphor is scanned with stimulating rays, light emitted upon stimulation is detected and converted into an electric signal representation of the image.
Since the method of the present invention is based on different processing of pixels depending on whether or not they are part of a diagnostically irrelevant image zone, this diagnostically relevant or irrelevant zone is first to be determined.
A diagnostically irrelevant zone is defined as encompassing all image points within the image that are not comprised within a diagnostically relevant image zone, that can for example be defined under visual control on a monitor or that can be determined automatically as will be described hereafter.
Several methods have been developed for recognizing a diagnostically relevant (or irrelevant) region. In European patent application 523 771, a number of methods have been described for manually delineating the diagnostically relevant zone in a radiographic image.
Delineation a diagnostically relevant image zone can for example be performed by the radiologist e.g. on the image displayed on a monitor. Although a man-controlled way of operation is described hereinafter, it is clear that the functions set forth hereinafter may be readily automated.
Delineation of a diagnostically relevant image part(s) within the overall radiographic image may be determined according to any of the following methods.
In all four methods described hereinafter, the radiographic image is first visualized on the screen of the preview monitor, a moveable light mark is generated on the screen of said monitor, and the motion of said light mark is synchronized with the motion of a marking means of a coordinate identification device such as a coordinate pen.
Now, according to a first embodiment, the diagnostically relevant image zone is defined as comprising all image points, the coordinates whereof are comprised within a contour drawn by moving said light mark under visual control on the screen of the monitor.
According to a second embodiment, an image point is marked as the upper left corner point and another image point is marked as the lower right corner point of the diagnostically relevant image zone. The coordinates of both said image points are determined and a rectangle on the basis of said coordinates is defined. Thereupon the diagnostically relevant image zone is defined as comprising all image points the coordinates whereof are comprised within said rectangle.
According to a third alternative embodiment, one image point is marked as the center point and another image point is marked as the outer point of the diagnostically relevant image zone; after determining the coordinates of both said image points, the diagnostically relevant image zone is defined as comprising all image points the coordinates whereof are comprised within a circle, the center point whereof coincides with the image point marked as the center point of the diagnostically relevant image zone, and the radius whereof is defined by the vector distance between said center point and the other image point marked as the outer point of the diagnostically relevant image zone.
According to a fourth embodiment, various image points are marked as the corner points of the diagnostically relevant image zone, said zone being defined as comprising all image points the coordinates whereof are comprised within a polygon the corner points whereof coincide with the image points marked as corner points of the diagnostically relevant image zone.
Whereas the first and fourth methods described have the advantage that the radiologist may define very accurately the diagnostically relevant image part, and the second and third methods offer the advantage of ease of operation. It suffices to mark only two image points for defining the diagnostically relevant image part. Whereas the third method is suitable for being used when radiographic images of e.g. the skull have been taken, the second method can advantageously be used for radiographs e.g. of the chest.
The above methods of defining the diagnostically relevant parts in a radiographic image can be used either alone or in combination with each other, in case a radiographic image would comprise e.g. more than one diagnostically relevant zone.
The method applied in a preferred embodiment is a method for automatically determining the region of interest. This method has been described in extenso in the published European patent application 610 605 (published Aug. 17, 1994). The contents of this application is hereby incorporated by reference.
In according with the method disclosed in EP 610 605 the location of a signal/shadow boundary in an X-ray image represented in a digital signal representation is first determined.
Then a different binary value is allocated to pixels within the determined signal shadow boundary than to pixels outside said signal shadow boundary.
The signal/shadow boundary is obtained by performing the following method steps:
i) Extracting low-level primitives from the image {X(i,j)}, more specifically said low level primitives are lines,
ii) Forming a reduced number of intermediate-level primitives from the low-level primitives, said intermediate level primitives are line-clusters,
iii) Building hypotheses as to the location of the signal-shadow boundary from combinations of intermediate-level primitives, during which each combination is subjected to intermediate-level tests,
iv) rejecting or accepting partial or complete hypotheses upon the result of said tests,
iv) Performing high-level verification tests on each hypothesis, whereupon hypotheses are rejected, or accepted at some cost, and
v) Selecting the hypothesis with the least cost.
As a result of this method, the information as to whether or not a pixel was part of the region of interest is given in the form of an overlay image. This overlay image is a low resolution binary image comprising labels which identify a pixel as belonging to the region of interest or not.
Blocking artifacts which were caused by the particular nature of the overlay image obtained by application of the method disclosed in European patent application 610 605, namely by the fact that the overlay image is a low resolution binary image, are avoided by transforming the binary overlay into a multiple valued overlay image through application of low pass filtering.
A gradual transition of the applied mapping transformations was provided for pixels outside the region of interest.
The slope and the intersect of the applied mapping transformation was controlled by the pixel value in the overlay image, being multiple valued instead of binary.
The applied mapping transformation has a maximal slope in the collimation material shadow region and is equal to the identity mapping in the diagnostically relevant image region. This is described in further detail hereafter.
This transformation can mathematically be expressed as follows:
g(x,y)=[1+C(x,y)(axe2x88x921)]f(x,y)+C(x,y)(1xe2x88x92a)fmax
wherein f(x,y) are pixel values before transformation, c(x,y) equals zero for pixels within the region of interest and equals 1 for pixels inside the collimation shadow region and has an intermediate value in the transition zone, and xe2x80x98axe2x80x99 represents the mapping slope within the collimation shadow region.
If xe2x80x98axe2x80x99 equals 1 then g(x,y) is equal to f(x,y) everywhere in the image, in other words there is no distinction between pixels within the region of interest or within the collimation shadow zone.
If for example xe2x80x98axe2x80x99 equals 1/3; then in the region of interest c(x,y) is equal to zero and g(x,y) is equal to f(x,y) and outside the region of interest c(x,y) equals 1 so that g(x,y)=af(x,y)+(1xe2x88x92a)fmax.